1. Field of the Invention
The present invention relates to a rubber composition containing silica and a silane coupling agent, more specifically to a rubber composition blended with silica capable of improving a rate of the reaction between silica and a silane coupling agent and improving a stability of rubber at a processing step.
2. Description of the Related Art
In recent years, low fuel consumption is strongly requested to be compatible with wet grip for a tread part of a tire. In order to solve this, it is rapidly increasing to use a rubber composition blended with silica for a tread.
While this rubber composition blended with silica for a tire tread has the advantage described above, disadvantages of a degradation in the processability and an increase in the cost come out as the blending amount of silica increases.
In general, silica has a poor affinity with polymers and is inferior in a reinforcing property. In addition thereto, since the dispersion becomes poor, the Mooney viscosity value is increased and hence the extrusion processability is deteriorated. Accordingly, a silane coupling agent is used to solve the problems of improving the dispersibility and the processability (Japanese Patent Application Laid-Open No. Hei 8-259736).
However, the existing situation is that since a silane coupling agent has a small efficiency of coupling action, an increased amount thereof is blended in order to make up for it. Further, there is a problem in that since a coupling agent is expensive, an increased blending amount of the coupling agent leads to a large increase in the cost.
A silane coupling agent containing an alkoxy group is disclosed in Japanese Patent Application Laid-Open No. Hei 8-259736. Silica is reacted with an alkoxy group-containing silane coupling agent during kneading to produce alcohol as a byproduct. Usually, the reaction is not completed and further goes on during the subsequent extrusion processing to produce alcohol. Thus, bubbles and blisters formed by allowing bubbles to grow into cavities are produced, so that a dimensional stability is damaged or the processing speed is obliged to be lowered in order to inhibit the blisters from being produced.
If the temperature in the kneading is elevated, the reaction shall be more accelerated. However, if the temperature is elevated too much, another inconvenience such as gelation is caused by vulcanization due to sulfur originating from a coupling agent at the same time as the reaction goes on, and therefore there is a limit for elevating the temperature.
If the blending amount of a coupling agent is reduced in order to decrease the production of alcohol, caused are inconveniences such as an increase in the viscosity and a reduction in the reinforcing property due to poor dispersion of silica.
Further, disclosed in Japanese Patent Application Laid-Open No. Hei 9-118784 (U.S. Pat. No. 5,616,655) is a processing method of a rubber composition characterized by mechanically mixing a rubber composition comprising 100 parts by weight of a sulfur-curing elastomer, 10 to 250 parts by weight of fine particulate precipitated silica, an organic silicon compound (silane coupling agent) having a specific structure of 0.01 to 1.0 part by weight per part by weight of said silica, and 10 to 250 parts by weight of sodium thiosulfate.pentahydrate (Na.sub.2 S.sub.2 O.sub.3.5H.sub.2 O) at a rubber temperature falling in the range of 140 to 190.degree. C. for 1 to 20 minutes.
The technology disclosed in this publication is a technology close to that of the present invention, and the object thereof is to improve a coupling efficiency of a silane coupling agent by adding sodium thiosulfate.pentahydrate to shorten the blending/processing time at a so-called nonproductive mixing step without sacrificing the properties of the finished product. In this method, an increase in the viscosity and a reduction in the scorch (prevulcanization of rubber) time are notably observed, and therefore it is not suited to practical use.
On the other hand, disclosed are a rubber component blended with an inorganic salt hydrate in Japanese Patent Application Laid-Open No. Hei 9-216968 and compositions blended with boric acid compounds in EP-A065476 and EP-A208505. However, all of these Patent Applications are to improve the adhesive properties and relate to compositions containing no silica-silane coupling agent.
Further, tin (Sn) compounds such as a tin salt of fatty acid are known as a curing catalyst for silicone rubber. However, such Sn compounds accelerate not only a reaction between silica and a silane coupling agent but also an unpreferable reaction between silane coupling agents themselves. Accordingly, bonds between the polymers and between the silicas are produced to elevate the viscosity of the rubber, and therefore they are not suited to practical use after all.